Hi everyone,
My white crowntail has a grey patch on his mouth pretty much since we got him from the LFS. Didn't really notice it when he was at the fish store though.

Anyways, he's had it for more than a month now and it doesn't seem to affect him at all. Health is great, playful, flares open when I show a little miss nemo toy on the outside of the tank, loves eating (pellets, flakes, mysis shrimps, blood worms, mosquito larvae) and inspecting all over the gravel for any food remains.

Below is a photo that I took of him in his QT maybe a week after I noticed it.

He's back in his filtered, heater and light equipped tank (14L), with about 30% weekly water change using RODI water.
Until now there's no change to the size of the grey thingy and his behaviour's still great.

In the QT I did 100% water changes everyday with aquarium salt dosed in as well as Melafix. Did this for about a week but nothing changed?

I can't tell much by the pic or not seeing what you are talking about-maybe get a different angle on the pic, however, since he is otherwise acting okay, eating, swimming and responding to you...I wouldn't worry about it-just observe for any changes.

His eyes don't look right to me.. popeye. I still can't really see much on his lip. I see maybe a different colored spot on the one side.. I'll leave that to OFL

You can't use RO water without kh buffers and readding minerals.. something like Kent's RO Right or Seachem's Alkaline Buffer and Equilibrium. You will also need to use test kits for gh and kh and test to make sure you have the right mix. Once you do, you must do it EXACTLY the same every time.

RO water is okay to use in small part, so long as you mix it -exact- each time.. not just topping off whenever it looks like it should be.. I wouldn't for a marine tank at all. they need even more TDS than fresh water fish.. and mixing in RO will change water parameters that will cause the fish stress and if it's done too fast it could even kill them. RO water is completely devoid of all minerals and electrolytes necessary for fish health. You should share this with your brother.

For example, some people with very high ph tap water will cut it 50/50 with RO water. If you do this it's VERY important to mix it EXACTLY the same every time before you add it to the tank. Because otherwise fluctuations in water parameters can lead to fish stress, illness, and even death.

As long as your tap water ph is below 9 and there's nothing odd about your tap water that would make it bad for fish, I would just use it.

When switching from RO to tap, you should do this very slowly. Try mixing it like 25% tap and 75% RO. Then your next water change you can do 50/50 and on up. Even with this slow mix I would do a careful acclimation by floating inside a little plastic cup in the new water for an hour while you slowly add a couple tablespoons of new water into his cup every 10 minutes.